A main focus of the contemporary semiconductor industry is the creation of smaller and more efficient memory devices and memory modules. These efforts are often frustrated by cross talk and signal noise. Cross talk is an inductive effect which can arise when a variable current flows through a conductor. Variable current creates a corresponding variable magnetic field surrounding the conductor capable of inducing a disruptive signal in any adjacent conductors passing through the magnetic field. As a consequence, the placement of conductors must be carefully engineered in order to maintain suitable distances of separation between the conductors to minimize the effects of cross talk.
Similarly, noise is interference that results in the corruption of electrical signal integrity. Noise can be caused by any of a variety of different sources, including radio waves and adjacent electrical wires or magnetic fields. Common techniques for ameliorating noise include shielding conductors and spacing conductors from each other, and from other electrical components.
Overall, the necessity of such careful considerations in shielding and spreading out conductors to minimize the effects of cross talk and noise complicates efforts to create cheaper and smaller memory devices.
A common memory device, such as a dynamic random access memory (DRAM), includes a semiconductor on which electronic circuitry (i.e., an integrated circuit) is fabricated. The chip is physically and electrically attached to a chip package, which is a protective container, such as a plastic dual-in-line package (DIP) or printed circuit board to which the chip is coupled. The chip is typically electrically coupled to the chip package by forming electrical connections between bonding pads on the chip and leads or pins on the chip package.
As the functionality of memory devices increases, the complexity of the electronic circuitry typically increases along with the required number of pins on the chip package required to support this increased functionality. For example, as the storage capacity of a DRAM increases, more address pins are required to access the data stored in the DRAM. To couple the DRAM to a circuit board, each pin must be electrically coupled to a conductive trace in a control, address or data bus. As the number of pins on the DRAM increases, the corresponding spacing between pins and conductive traces decreases, which heightens the potential for cross talk and noise on the control, address and data busses.
In a typical application, a plurality of DRAMs are mounted on a circuit board to form a memory module. Each DRAM receives address and control signals through address and control terminals on the circuit board, and has a data bus coupled to a corresponding data terminals on the circuit board. Typically, the memory module has a data bus that is M bits wide, where M is an integer multiple of N, which is the width of the data bus of each DRAM. Each DRAM on the module provides N of the M bits in response to common address and control signals applied to all DRAMs on the module. For example, a typical memory module includes 8 DRAMs each having an 8 bit wide data bus to form a 64 bit wide data bus on the memory module. Another typical memory module includes 9 DRAMs, each having an 8 bit wide data bus to form a 72 bit wide data bus on the memory module with 8 bits that function as error checking and correction bits.
FIG. 1 is a simplified block diagram of a DRAM 100 including an address decoder 102 that receives address bits A0-AX on an address bus ADDR and decodes these address bits and applies decoded address signals 104 to a memory-cell array 106. The memory-cell array 106 includes a plurality of memory cells (not shown) arranged in rows and columns, each memory cell storing a bit of data. The data stored in the memory cells is accessed in response to the decoded address signals 104 from the address decoder 102. A read/write circuit 108 is coupled to the memory-cell array 106 through an internal data path 110 and is coupled to an external data bus DATA of the DRAM 100. In the example of FIG. 1, the data bus DATA includes 8 external terminals over which data bits DQ0-7 are transferred to and from the DRAM 100.
As discussed above, however, the data bus DATA can include more terminals, such as 32 terminals, to transfer a corresponding number of data bits. As the number of terminals increases, the spacing between the terminals on the data bus DATA decreases, increasing the risk of cross talk and noise. The DRAM 100 also includes control logic 112 that receives a plurality of control signals applied on an external control bus CONT. In response to the control signals, the control logic 112 generates a plurality of control and timing signals 114 to control the timing and operation of the address decoder 102, memory-cell array 106, and read/write circuit 108 during operation of the DRAM 100.
In operation, an external circuit (not shown) such as a memory controller applies address, control, and data signals to the DRAM 100 over the address bus ADDR, control bus CONT, and data bus DATA, respectively, to control the operation of the DRAM. During read operations, the external circuit applies a read command to the DRAM 100 in the form of appropriate address signals on the address bus ADDR and control signals on the control bus CONT. In response to the applied address signals, the address decoder 102 accesses addressed memory cells in the memory-cell array 106 and applies the read data stored in the addressed memory cells over the internal data path 110 to the read/write circuit 108 which, in turn, places the read data on the data bus DATA as read data bits DQ0-7. The control logic 112 generates the appropriate control and timing signals 114 to control the address decoder 102, memory-cell array 106, and read/write circuit 108 during the read operation.
During write operations, the external circuit applies a write command to the DRAM 100 in the form of appropriate address signals and control signals on the ADDR and CONT buses, respectively, and also applies write data bits DQ0-7 on the data bus DATA. Once again, in response to the applied address signals, the address decoder 102 accesses the addressed memory cells in the memory-cell array 106. The read/write circuit 108 transfers the applied write data bits DQ0-7 over the internal data path 110 and into the addressed memory cells in the memory-cell array 106. The control logic 112 operates during write operations to generate the appropriate control and timing signals 114 to control the address decoder 102, memory-cell array 106, and read/write circuit 108.
In both of the read and the write operations, considerable potential for cross talk and noise exists as electrical signals are coupled to and from the DRAM through the address bus ADDR, the control bus CONT, and the data bus DATA. This is exacerbated as the number of terminals on these busses increases, and the spacing between the terminals is necessarily decreased.
There is thus a need to decrease the density of busses coupling electrical signals to and from the pins of a DRAM to lessen the potential for cross talk and noise, without reducing the number of pins on the DRAM.